1. Field of the Invention
The invention relates to an ultrasonic gas flowmeter including at least one transmitting sound transducer, one receiving sound transducer, transmission and reception electronics, and a device to measure exhaust flows of internal combustion engines, as well as a method to determine the flow of gases whereby the mean flow velocity is determined and whereby the flowing quantity of gas is also determined with a highly synchronized resolution from the transit time of the acoustic signals between transmitter and receiver.
2. The Prior Art
Such ultrasonic flowmeters are known for fluids and gases and they are the subject of various publications and professional articles. They use the so-called dragging effect, namely in that the velocity in the direction of the beam of acoustic signals in fluids is not only determined by the orientation of the acoustic transmitter and the sound velocity of the (static) medium, but these acoustic signals are dependent, among other things, on the flow velocity of the fluid medium. At least two transit times along at least two measurement paths are measured whereby at least one path must be oriented parallel or at an angle to the flow direction, upstream or downstream. The course of transit relative to one or other measuring paths can be transverse, shifted parallel or converging parallel as well.
The at least two transit times of the transmitted sound characteristics in the region of the flowing medium are determined from two measured time differences between transmitting time and receiving time. A mean flow velocity can be determined from the transit times, which results in the searched volume flow data with the aid of the known diameter of the pipe with the flowing medium.
The mass flow of the flowing gas can be calculated from the volume flow if the density of the gas is known. However, this is generally not known, especially in case of a pulsating gas flow and overlaying pressure pulsations and high temperature fluctuations. It is now a possibility to determine also the effective propagation velocity of the sound from at least two transit times of the sound, which is approximately the sound velocity of the static gas, and at the same time to measure the actual gas pressure and possibly also the actual gas temperature—simultaneously to the measuring of transit times—and to take these values into consideration in the evaluation.
However, this known method has currently limited applications. Based on the known equations for the ideal gas there is additionally necessary, as a result of an accurate determination of the gas mass-flow, either the knowledge of the adiabatic coefficient of the gas—that is the ratio of the specific heat capacity at constant pressure at constant volume—or the knowledge of the molar mass. However, these values are not always known and they are not constant over time in case of an exhaust gas from a variably occurring combustion.
The values of the mean flow velocity and the sound velocity are obtained from the transit times and the course of the flow in the volume, which passes the sound paths. They respectively represent a determined value of the sound path and the transit time, while the determined flow velocity relative to the diameter of the pipe is of importance in the determination of the flowing gas quantity. Since these two pieces of information do not give generally the same results, complicated systems were envisioned to minimize the influence of the flow profile on the sound paths and on the values of flow and sound velocity, which are determined from the transit times. For example, it was proposed to position several ultrasonic transducers in such a manner that the flow velocity determined from the transit times corresponds to the velocity determined from the diameter of the pipe. In addition, special sound paths in the vicinity of the pipe wall were proposed, especially for large pipe diameters. Through a suitable but costly arrangement and evaluation, it was to be ensured that the flow velocity, determined from the transit times, corresponds to the velocity detected relative to the pipe diameter.
It is further known that an error correction can be performed in the form of a calibration constant to consider the flow profile existing in the medium, which is sensible, nevertheless, only in case of a time-constant flow characteristic but not in case of a non-stationary and pulsating flow.
One disadvantage of the known device and method is especially the fact that they are often and wrongly based on a linear course of the sound path in the pipe with flowing medium.
In fact, the azimuthal flows in the measuring pipe have a well-known large influence on the respective sound path and thereby and influence on the measurement results of an ultrasonic gas flowmeter. Flow-shaping devices are suggested to be installed inside the measuring pipe as a remedy against such rotational velocities, i.e. flow rectifiers or stream-lining devices in the form of lamellas or thin tubing.
However, it is generally not taken into consideration that sound breaks occur based on an axial flow profile besides the altering drift of the sound. Starting from zero at the ultrasonic transducer diaphragm and near the pocket of the transducer, which is mounted to the measuring pipe at an angle, the flow velocity is even or smooth up to the maximum velocity at about the center of the pipe. A deviation of the local sound impedance occurs dependent on the local sound velocity relative to the flow velocity whose gradient causes the breaking of sound.
The additional breaking as a result of a temperature profile in the flowing gas, which is also not considered in prior art, is of special significance. Especially at a temperature difference between the measuring pipe and the medium there can be a sound path largely deviating from the linear propagation.
It may occur in an extreme case that the sound emitted by the aligned transmitter and receiver is made to drift or is broken to such a high degree that it does not reach the receiver at all and a measurement of the transit time is no longer possible thereby. A circumstance like this can be observed, for example, in the measurement of the quantity of exhaust gas in an internal combustion engine. At sudden load changes from idling to full load, there might occur high flow velocity and temperature differences of 300° C. between the flowing exhaust gas and the pipe, for example, which can lead to the formation of extreme flow and temperature profiles and to the deflection of the sound from the linear propagation, depending on the pipe dimension of up to a few centimeters.
Drifting and breaking of the sound in the non-stationary gas flow, which has a non-stationary temperature profile, are also the cause that now only one part of the maximal detectable amplitude, dependent on the temporarily existing flow characteristic, reaches the receiver and is measured thereby. In addition, the transmission of sound can be extremely distorted and damped based on the local vorticity and pressure fluctuations up to cavitation effects. These highly fluctuating interruptions lead to a strong influence on the amplitude and signal form of the two reception signals, which results in placing high demands on the evaluation method, the ultrasonic transducer, and on the entire arrangement. Based on these effects, the usable measuring range of the flowmeter is clearly limited and the evaluation of data is made more difficult. Furthermore, the employment of the flowmeter on an engine test bench represents a difficult surrounding for the sensor electronics relative to the electromagnetic compatibility (EMV). Traditional systems and evaluation methods, e.g. cross-correlation with a stored reference signal (EP 0 797 105 A2), or methods using threshold sampling (DE 196 36 945 A1), cannot satisfy all these commands.
The flow velocity to be measured in the exhaust pipe may include a wide range of values, mainly when the usual standard diameters are used for the measuring pipe, independent from the size of the engine. There are in fact proposals relative to maximizing the measuring range proposing a special mechanical alignment of the transducer with a specific correction angle (K. S. Mylvaganam, “High-Rangeability Ultrasonic Gas Flowmeter for Monitoring Flare Gas,” IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control, vol. 36, pp. 144–149, 1989.) However, this requires higher manufacturing costs for the measuring pipe and does not make possible constant adaptation to different flow velocities and different temperature profiles.
The exhaust gas temperatures can be from −40° C. to approximately 1,000° C., depending on the operational condition of the internal combustion engine and the position of the flowmeters in the exhaust train (e.g. at a cold start of the engine in the environmental chamber.) Current devices (i.e. from Sick AG) are highly limited relative to the maximal admissible exhaust gas temperature (200° C. ) because of the use of piezo-ceramic ultrasonic transducers.
Deciding is also the fact that the exhaust gas temperature of an internal combustion engine can change rapidly and to a high degree, e.g., at a load change in engine operation at full load during pulling operation. Based on this large and rapid change of the flow characteristics, there may also occur unpredictable reflections and overlaying of the actual reception signal together with parasitic sound signals, which would lead to false results during the use of traditional evaluation methods.
Known ultrasonic gas flowmeters have the difficulties to detect sufficiently accurate the pulsations of pressure and flow velocities existing in the exhaust gas flow to a high extent. Based on the scanning theorem, there is namely the requirement for the avoidance of measurement artifacts (aliasing) to scan the signal at such a high frequency that the scanning frequency is at least twice as high as the frequency of the signal component existing in the signal that has the highest frequency. An exhaust gas quantity sensor must therefore have also a correspondingly high measurement repetition rate. One has to assume for automobile engines, for example, a repetition frequency rate of at least 3 kHz, depending on the position of the mass flow sensor in the exhaust train. Commercially available devices (e.g. Sick AG) operate with a measurement repetition frequency of maximal 30 Hz.
Based on the condition existing in the exhaust pipe, the evaluation method used in the exhaust gas quantity sensor should have an adequate possibility for plausibility controls of the detected flow values.
The employment of capacitive ultrasonic transducers represents a basic improvement for ultrasonic flowmeter for gases as it was already proposed in general (I. J. O'Sullivan and W. M. D. Wright, “Ultrasonic measurement of gas flow using electrostatic transducers,” Elsevier Ultrasonics, vol. 40, pp. 407–411, 2002.) It is not known, however, how the number of the aforementioned problems can be solved. In particular, there were not disclosed any suitable evaluation methods, transducers and arrangements for the employment in the exhaust train of an internal combustion engine.
A general difficulty with ultrasonic transmission and reception transducers for gases is to send sufficient acoustic energy into the medium and to be able to obtain a sufficiently strong electric reception signal from the received acoustic energy. Up to now, piezoelectric ultrasonic transducers were practically exclusively employed as transducers that are distinguished by a compact structure made of a solid-state material. The large difference in the acoustic-wave resistance between the gas medium, on the one hand, and the material of the sound transducer, on the other hand, has a disruptive effect since solid-state materials have a specific wave resistance that is 100,000-fold higher than the one of gases. This means that most of the acoustic energy is reflected from the bordering surface of the transducer to the gas medium and only a small portion is transmitted. These transducers have therefore an extremely low sensitivity in high frequency ranges for transmission or for reception as well.
The same characteristic has the result that the piezoelectric sound transducer can easily described as solid-state acoustic resonators with a characteristic natural frequency and having a relatively high oscillating quality or having a narrow-band frequency characteristic. This fact is intentionally exploited to obtain a sufficiently high sensitivity: In the range of its natural frequency or its resonance frequency there is an acceptable high sensitivity based namely on the resonant rise, even though the sensitivity drops to essentially unusable low values outside this narrow-band frequency range. Of course, there is related to the frequency characteristic of the high-quality resonator also a long-term build-up and decay behavior of the transducer, which causes difficulties again with the accurate transit time measurement and which leads thus to inaccuracies and a low sampling rate during flow measurement.
Various attempts have been disclosed to improve the situation. It has been tried to damp the back echo in the piezoelectric transducer through sound-absorbing layers, so-called “backing layers” to increase the bandwidth to some degree in this manner—this was accomplished, however, in exchange for a loss in sensitivity. Layers were arranged on the front of the transducer for so-called impedance matching to the wave resistance of the gas medium, however, with little success. The transducer element itself was made also of a composite wherein piezoelectric rods are embedded in a plastic polymer matrix so that the wave resistance of this element of composite is lowered and “clean” vibration modes are achieved at the same time. Signal-analytical methods were developed to achieve accurate transit-time measurements in spite of the occurrences of build-up and decay. Even with all these efforts, the efficiency of ultrasonic flow measurement remained limited mainly because of the narrow band of the employed transducer, especially the piezoelectric ultrasonic transducer.
An additional disadvantage of these transducers exists in their limited temperature stability. Its metallic coated diaphragm stretched over an electrically conductive substrate forms at the same time the insulation layer of the electric capacitor whereby the plastic foils or silicone nitride usually used as dielectric diaphragm material are not sufficient for the temperature requirements for the quantity measurement of the exhaust gas (D. A. Hutchins, D. W. Schindel, A. G. Bashford, and W. M. D. Wright, “Advances in ultrasonic electrostatic transduction,” ElsevierUltrasonics, vol. 36, 1998.) Even the so-called electret transducers, which have a permanently polarized dielectric diaphragm, do not have sufficient temperature stability, i.e. a Teflon-type polymer diaphragm with inserted electric charge carriers (electrons).
A particular disadvantage with traditional ultrasonic flowmeters with oblique irradiation by ultrasonic waves is the angled position of the transducer to the pipe wall, which is required in traditional transducers. The thereby developing recesses or pockets cause an advanced transit time of the ultrasonic wave that would have to be considered in the evaluation of the transit times. In addition, flow vortexes are induced in the recesses and in the flow that can cause falsification of the measured values. The vortexes additionally increase the problem of the deposit of particles transported along with the flow. Particles deposited on the transducer diaphragm can greatly change the transmission characteristics of the transducers. The disclosed proposal for a remedy could not sufficiently solve the problem, for instance, a screen stretched over a recess that is to be permeable for the ultrasound but impermeable for the flow—or the aeration of the recess with clean air.
The use of capacitive ultrasonic transducers does not only offer advantages. Improvements in circuitry would be desirable relative to the polarization voltage, which has a considerable co-effect on the electrical and mechanical operational center of the transducer. The required polarization voltage of 100 to 200 volts, for example, is usually established for the transducer capacity via a high-ohmic electric resistor. The resulting electrostatic force causes, on the one hand, the flat placement of the diaphragm onto the textured back plate and, on the other hand, a linear transducer characteristic, which means, a transducer sensitivity that is almost independent from the amplitude of the electric transmission signal or the acoustic reception signal. However, the polarization voltage prevents also the simple use of circuitry concepts common in piezo-ceramic transducers or electret transducers, namely the electrometer and charge amplifiers that relate directly to the mass potential.
The charge amplifier and the electrometer amplifier would both be advantageous and nearly of the same value as reception amplifier relative to the achievable SNR (signal-to-noise ratio); however, in contrast to the electrometer circuit, the charge amplifier makes a greater bandwidth possible, which can be advantageously used especially in high-frequency applications such as optical data transmission with photo diodes or with ultrasound. The large bandwidth is the result of the fact that the parasitic capacity of the transducer and of the connecting cable does not have to be recharged with the signal voltage in case of the charge amplifier concept. The operational amplifier creates in fact zero voltage at its inverting input so that the voltage in the transducers and in the existing parasitic components remains exceedingly small to the point of disappearance.
However, a charge amplifier in the traditional form is not possible because of the electric polarization voltage necessary in capacitive ultrasonic transducers. The electrically pre-charged transducer has been coupled up to now to the charge amplifier acting against the mass potential via a voltage-proof coupling capacitor. A “pure” charge amplifier operation of the capacitive ultrasonic transducer is no longer possible with all its advantages. The transducer lies no longer directly at the virtual zero-point of the operational amplifier, which reduces the bandwidth that can be achieved. A very large coupling capacitor would put too much load on the transducer—but with small transducers, the total sensitivity of the transducer plus the amplifier or their group transit time would not be defined enough based on the manufacturing-related fluctuations of the transducer capacity. In case of the ultrasonic flow measurements by means of transit time detections at two reception channels, two respective amplifiers would have to be calibrated in the rule since any asymmetry would lead directly to a time error based on the group transit times, which must be considered to be extremely problematic. The same applies to the connecting cable since achieving of complete symmetry of the two connecting cables is difficult and costly in a final configuration of the apparatus.
As a conceivable alternative thereto, one would have to mention also the electric impedance conversion occurring directly on the transducer in the housing. However, this is to be discounted because of the lack of space and in view of the high temperatures of the exhaust gases to be measured.
The aforementioned disadvantages of the state-of-the-art are of special significance in the measurement of exhaust gas flows of internal combustion engines and they can prevent the realization of advantageous devices in the measuring technology having gas flowmeters in the exhaust gas system (see for example WO 02/42730 A2 of PCT/AT01/00371), particularly in hot and strongly pulsating regions. With the available gas flowmeters there can be performed, nevertheless, some applications in the automotive measuring technology such as, for example, the measuring of blow-by gases (leaking gas from the crankcase.) However, the possible applications are very limited, especially in the exhaust gas analysis on the engine and drive-train test bench as well as on the roller-type dynamometer for vehicles, or in vehicles on the road.
It is the object of the present invention to overcome the aforementioned difficulties and to provide an ultrasonic gas flowmeter having considerably improved capacities, particularly in view of temperature stability and the reduction and consideration of an existing temperature profile.
An additional object of the invention is an improvement of the evaluation method for more accurate and dependable detection of the volume flow or the mass flow of gases, especially highly dynamic flows.